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TRANSFORMER INRUSH PHENOMENON 1. INTRODUCTION It is recommended that differential protection be used for the protection of transformers of 10MVA (self-cooled) and higher. Differential protection generally is considered the best protection for transformers. However, inrush current due to transformer energization exists mostly only in one winding of the transformer; therefore, the relay sees the energization condition as a fault. To improve security while maintaining the required levels of dependability, many restraint methods have been proposed to block the operation of the differential element due to the inrush current. It is very well known that a transformer will experience magnetizing inrush current during energization. Inrush current occurs in a transformer whenever the residual flux does not match the instantaneous value of steady-state flux. PEPS EEE DEPARTMENT AJCE 1 TRANSFORMER INRUSH PHENOMENON 2. MAGNETIZING INRUSH CURRENTS Transformer inrush is described as the phenomenon: "When a transformer is suddenly energized with full system voltage , a random saturation phenomenon may occur, which is usually referred to as an inrush current" The inrush current has a high degree of asymmetry and is harmonically rich due to it being created by saturation of the transformer's magnetic circuit. In addition, there is a large DC offset component which contributes significantly to the peak component. The inrush current typically lasts for tens of cycles and as such has a prolonged effect on the voltage of the network it is being energized from. The degree to which it depresses the network voltage is dependent on the short-circuit strength of the network relative to the rating and the inrush characteristic of the transformer in question Time (sec) FIGURE 2.1 –TYPICAL TRANSFORMER INRUSH CURRENT [2] PEPS EEE DEPARTMENT AJCE 2 TRANSFORMER INRUSH PHENOMENON 3. CAUSES OF INRUSH CURRENT Inrush current in transformers results from any sudden change of the magnetizing voltage. Although usually considered a result of energizing a transformer, the magnetizing inrush may be also caused by 1. External faults, 2. Voltage recovery after clearing an external fault, 3. Out-of-phase synchronizing of a connected generator. Since the magnetizing branch representing the core appears as a shunt element in the transformer equivalent circuit, the magnetizing current upsets the balance between the currents at the transformer terminals, and is therefore experienced by the differential relay as a "false" differential current. The relay, however, must remain stable during inrush conditions. In addition, from the standpoint of the transformer life-time, tripping- out during inrush conditions is a very undesirable situation (breaking a current of a pure inductive nature generates high over voltage that may jeopardize the insulation of a transformer and be an indirect cause of an internal fault). PEPS EEE DEPARTMENT AJCE 3 TRANSFORMER INRUSH PHENOMENON 4. FACTORS INFLUENCING INRUSH CURRENTS Size of a transformer The peak values of magnetizing inrush currents are higher for a smaller transformers while the duration of this current is longer for larger transformers. The time constant for the decaying current is in the range of 0.1 of a second for small transformers(100kVA and below) and in the range of 1 second for larger units. Impedance of the system from which a transformer is energized The inrush current is higher when the transformer is energized from a powerful system. Moreover, the total resistance seen from the equivalent source to the magnetizing branch contributes to the damping of the current. Therefore, transformers located closer to the generating plants display inrush currents lasting much longer than transformers installed. Magnetic properties of the core material The magnetizing inrush is more severe when the saturation flux density of the core is low. Designers usually work with flux densities of 1.5 to 1.75 Tesla. Transformers operating closer to the latter value display lower inrush current. Remanence in the core Under the most unfavorable combination of the voltage phase and the sign of the remanent flux, higher remanent flux results in higher inrush currents. The residual flux densities may be as high as 1.3 to 1.7 Tesla. Moment when a transformer is switched in The highest values of magnetizing inrush current occurs when the transformer is switched at the zero transition of the winding voltage, and when in addition, the new forced flux assumes the same direction as the flux left in the core. In general, however , the magnitude of the inrush current is a random factor and depends on the point of the PEPS EEE DEPARTMENT AJCE 4 TRANSFORMER INRUSH PHENOMENON voltage waveform at which the switchgear closes, as well as on the sign and value of the residual flux. It is approximated that every fifth or sixth energizing of a power transformer results in considerably high values of the inrush current. Way a transformer is switched in The maximum inrush current is influenced by the cross-sectional area between the core and the winding which is energized. Higher values of inrush current are observed whwn the inner (having smaller diameter) winding is energized first. It is approximated that for transformers with oriented core steel, the inrush current may reach 5-10 times the rated value when the outer winding is switched –in first, and 10-20 times the rated value when the inner winding is energized first. Due to the insulation considerations, the lower voltage winding is usually wound closer to the core, and therefore, energizing of the lower voltage winding generates higher inrush currents. Some transformers may be equipped with special switchgear which allows switching-in via a certain resistance. The resistance reduces the magnitude of inrush currents and substantially increases their damping. In such a case, the operating requirements for the differential protection are much more relaxed. In contrast, when a transformer is equipped with an air-type switch, then arcing of the switch may result in successive half cycles of magnetizing voltage of the same polarity. The consecutive same polarity peaks cumulate the residual flux and reflect in a more and more severe inrush current. This creates extreme conditions for transformer protection. PEPS EEE DEPARTMENT AJCE 5 TRANSFORMER INRUSH PHENOMENON 5.REDUCTION TECHNIQUES Probably every utility has experienced a false operation of a differential relay when energizing a transformer bank. Over the years, many different methods of preventing differential relay operation on inrush have been implemented. 1. Ignore it. That is, if it can be verified that there was actually no fault (via visual inspection, no sudden pressure relay operation, no oscillograph operations in the area, etc.) in the transformer, the bank is simply reenergized. Should the bank trip again, in some cases it has been reported that the differential trip cutout switch would be opened, the bank energized, the differential relay contacts verified to be open, and the cutout switch closed. 2. Desensitize the relay by: present on energization. On inrush, it is assumed that the bus voltage will not drop appreciably; the auxiliary relay will be picked up, restraining the differential element from operating. If a fault exists, the voltage will drop, the auxiliary relay drops out, allowing the differential element to operate. 3. Use slow-speed induction-type relays with long time and high current settings. 4. Power differential method - This method is based on the idea that the average power drawn by a power transformer is almost zero on inrush, while during a fault the average power is significantly higher. 5. Rectifier relay - This method takes advantage of the fact that magnetizing inrush current is in effect a half-frequency wave. Relays based on this method use rectifiers and have one element functioning on positive current and one on negative current. Both elements must operate in order to produce a trip. On inrush, the expectation is that one element only will operate, while on an internal fault, the waveform will be sinusoidal and both elements will operate. PEPS EEE DEPARTMENT AJCE 6 TRANSFORMER INRUSH PHENOMENON 6. A variation of method 5 is the method of measuring “dwell-time” of the current waveform, that is, how long it stays close to zero, indicating a full dc-offset, which it uses to declare an inrush condition. Such relays typically expect the dwell time to be at least ¼ of a cycle, and will restrain tripping if this is measured. 7. Another unique method uses the flux-current relationship of the transformer to provide restraint. 8. Harmonic current restraint - This is the most common method and is discussed in more detail below. An important feature of this inrush current is that it is evident that the currents are not pure fundamental frequency waveforms. Past research has shown that magnetizing inrush produces currents with a high second harmonic content, with relatively low third harmonic content. Relay designers have taken advantage of this fact, along with the fact that internal fault currents have relatively low harmonic content. Relays have been designed with fixed second harmonic restraint thresholds that will restrain tripping if the input currents have a certain level of harmonic current, and allow tripping if the second harmonic content is below that particular threshold. Different manufacturers chose different thresholds. PEPS EEE DEPARTMENT AJCE 7 TRANSFORMER INRUSH PHENOMENON 6.HARMONIC RESTRAINT OPERATION For EHV transformers, the relay current and time ratings necessary to ensure stability on the magnetizing inrush current caused by switching-in the transformer are not adequate for providing high speed protection. A high speed biased differential relay incorporating a harmonic restraint feature is immune to the magnetizing inrush current. The magnetizing inrush currents have high component of even and odd harmonics (about 63% of 2ND harmonics and 26.8% of 3RD harmonics) while harmonic component of short-circuit current is negligible. The use of these facts is made for restraining the relay from operation during initial current inrush. The harmonic restraint differential relay is sensitive to fault currents but is immune to the magnetizing currents. The operating coil of the relay carries only fundamental component of current only while the restraining coil carries the sum of the fundamental and harmonic components. The restraining coil is energized by a direct-current proportional to bias winding current as well as the direct current due to harmonics. Harmonic restraint is from the tuned circuit that allows only the fundamental component of current to enter the operating circuit. The dc and higher harmonics (mostly second harmonics) are diverted into the rectifier bridge feeding the restraining coil. The relay is adjusted so that it will not operate when the harmonic current exceeds 15% of the fundamental current. Both the dc and higher harmonics are of large magnitude during magnetizing inrush. The relay may fail to operate due to harmonic restraint feature if an internal fault has considerable harmonics, that may be present in the fault current itself due to an arc, or due to saturation of CT. Also, if a fault exists at the instant of energization of transformer harmonics present in the magnetizing current may prevent the operation of the relay. This problem can be overcome by providing instantaneous overcurrent relay in the differential circuit which is set above the maximum inrush current but will operate in less than one cycle on internal faults. Thus fast tripping is ensured for all internal faults. PEPS EEE DEPARTMENT AJCE 8 TRANSFORMER INRUSH PHENOMENON 7.HARMONIC RESTRAINT METHODS The majority of relay manufacturers use the terms of harmonic restraint, harmonic inhibit, or harmonic blocking interchangeably. Although there are some variations in implementation, the differential relay will not operate when Equation 1 - Second harmonic current - operating current In order to overcome the challenge of secure differential protection with low harmonic component in inrush current in new transformers, various harmonic restraint methods have been studied. 7.1 Per Phase Method This is the earliest and simplest harmonic restraint method. Equation applies to phase A, B and C separately. The restraint algorithm in each phase is independent and parallel. Since each phase has different residual flux and is energized at a different angle, each phase will most likely have different harmonic levels. When the second harmonic ratio for a particular phase is above a preset level, the percent differential operation on that phase will be inhibited. Both experience and analysis show that it is possible to have low second harmonic ratio for one phase during transformer energization. Differential operation for a phase with small second harmonic ratio may cause an undesirable trip for a three-phase transformer. The differential protection with per-phase harmonic restraint method is most dependent but least secure. PEPS EEE DEPARTMENT AJCE 9 TRANSFORMER INRUSH PHENOMENON 7.2 Cross-blocking Method The harmonic detection for cross-blocking method is the same as that of per phase method. The only difference is that the restraint signal from one phase will inhibit differential operation for all other phases. This improves security by allowing the phase with the low harmonic ratio to be cross-blocked by a phase with a higher ratio preventing possible false trips. However, inrush current is generally several times the rated current, and the insulation bears the most severe mechanical stress during energization. If there is an internal fault in one phase or two phases, the high second harmonic ratio in a healthy phase may block the percent differential protection until the fault spreads to all three phases. The differential protection with cross blocking harmonic method is very secure but least dependable. Two-out-of-three restraint method is a slight variation of the cross- blocking one. The blocking of differential operation will need at least two phases to detect sufficient harmonic level. The disadvantage of this variation basically stays the same as that of cross-blocking method. 7.3 Percent Average Blocking Method The harmonic ratio for a percent average blocking method is the average of the second harmonic ratio of the three phases Compared to the cross-blocking method and two-out-of-three method, this method improves the security of differential protection to a certain degree. The differential operation may be restrained when there is a true single phase fault during energization, provided that there are large harmonic ratios in the remaining healthy phases. This would cause a concern on the dependability in the differential protection. PEPS EEE DEPARTMENT AJCE 10 TRANSFORMER INRUSH PHENOMENON 7.4 Summing-type Harmonic Sharing Method A summing-type harmonic sharing restraint method greatly improves the dependability of the differential protection during an internal fault. The shared second harmonic component is defined as I 2nd sum = | I op2nd A| + |I op2nd B| + |I op2nd C| Equation 2 The second harmonic ratios are then calculated per phase based on the shared sum and the fundamental component of the operating current in each phase is, Equation 3 Equation 4 Equation 5 2nd Ratio A, B, C – Second harmonic ratio of phase A, B, C I 2nd sum - Sum of second harmonic current of three phases I op fund A, B, C – Operating current fundamental component of phase A, B, C PEPS EEE DEPARTMENT AJCE 11 TRANSFORMER INRUSH PHENOMENON The harmonic restraint with summing-type harmonic sharing is illustrated in the figure below FIGURE 7.1- RESTRAINT METHOD WITH SUMMING TYPE HARMONIC SHARING [1] In this method, the magnitudes of the second harmonic from three-phases are summed together to create a single harmonic signal, which is shared in the calculation of the second harmonic ratio in each phase. If there is an internal fault in one phase during energization, the large fundamental operating current would result in low harmonic ratio and, thus, no inhibit is generated from the faulted phase. Therefore, a three-phase transformer would be tripped during an internal fault. If one phase experiences a very low second harmonic ratio, the shared harmonic calculated from equation will be large and the second harmonic ratios calculated from equation will be large enough to restrain the differential protection from a false operation. PEPS EEE DEPARTMENT AJCE 12 TRANSFORMER INRUSH PHENOMENON 8. ANALYSIS OF INRUSH EVENT EXAMPLE A Low Second Harmonic Inrush Event Fig. illustrates a delta/star transformer connected to a radial distribution system. While this transformer is energized with load open, there is a station service transformer connected to the transformer secondary but outside the differential zone protection. When the main transformer is energized, the station service transformer also will be energized. This installation had problems with tripping during energization, and the user switched to a numeric relay. High voltage side CT is Y-connected and low voltage side CT is delta- connected for external angle compensation. A set of COMTRADE data files was downloaded from an energization event, which is illustrated in the figures below. FIGURE 8.1 - A DELTA/STAR TRANSFORMER [1] PEPS EEE DEPARTMENT AJCE 13 TRANSFORMER INRUSH PHENOMENON The per-phase second harmonic ratios for all three phases are illustrated in Figure There is very low second harmonic ratio in phase B. For differential protection using per-phase harmonic restraint, security likely is a problem. The low second harmonic ratio in phase B was probably the reason this transformer had problems with tripping during energization. CYCLES FIGURE 8.2 - PER-PHASE SECOND HARMONIC RATIO [1] PEPS EEE DEPARTMENT AJCE 14 TRANSFORMER INRUSH PHENOMENON When considering second harmonic cross-blocking for this application, the second harmonic ratios in phase A and C are large, as seen from Figure, differential tripping of phase B will be cross blocked. Figures indicate the result of applying average second harmonic ratio and summing-type harmonic sharing restraint methods. In both cases we observe the differential protection would be secure for this low harmonic energization. CYCLES FIGURE 8.3 - AVERAGE 2ND HARMONIC RATIOS [1] PEPS EEE DEPARTMENT AJCE 15 TRANSFORMER INRUSH PHENOMENON CYCLES FIGURE 8.4 - SECOND HARMONIC RATIO WITH SUMMING- TYPE HARMONIC SHARING [1] PEPS EEE DEPARTMENT AJCE 16 TRANSFORMER INRUSH PHENOMENON CONCLUSION Security is of concern in differential protection during transformer energization. Inrush current occurs mostly only in one side of the transformer and could cause a false differential trip. A common method is to use the second harmonic information in inrush current to secure differential protection when energizing transformers. An analysis of factors like residual flux, saturation flux, and energizing angle on the second harmonic ratio in inrush current is provided in this paper. Higher residual flux and/or lower saturation flux in transformers may result in high differential operating current and lower second harmonic ratio, which is likely to cause a security concern in differential protection during transformer energization. It is well known that modern transformers may experience very low second harmonic ratios. Per-phase harmonic restraint provides best dependability but worst security. Harmonic cross-blocking provides best security but worst dependability. Average-percent harmonic restraint may cause an unexpected blocking when there is a true fault during energization. The restraint with summing-type harmonic sharing provides very good dependability while maintaining the security for differential protection. PEPS EEE DEPARTMENT AJCE 17